A transmission system for a vehicle is suitably configured to deliver improved power transmitting efficiency, and improved fuel efficiency. Transmission mechanisms may include a manual transmission, a conventional stepped automatic transmission, a continuously variable transmission, a double clutch transmission, or another type of vehicle transmission. Such transmissions are typically used with conventional gasoline or diesel engine systems.
The transmission of power between engine components will, over time, result in wear of the components. In order to reduce this wear, certain sulfur-containing extreme pressure/antiwear additive components have been used in transmission fluids. For instance, U.S. Pat. No. 8,410,032 and US 2006/0025314 disclose transmission fluids containing substituted or unsubstituted compounds having a 2,5-dimercapto-1,3,4-thiadiazole core (hereinafter “DMTD” compounds”) as an extreme pressure/antiwear additive component.
U.S. Pat. Nos. 2,719,125 and 2,719,126 describe the preparation of various DMTD compounds. In these compounds, each hydrocarbyl group may be linked to the thiadiazole through multiple sulfur atoms. The hydrocarbon group may be aliphatic or aromatic, and may include cyclic and alicyclic hydrocarbyl groups, as well as aralkyl, aryl and alkaryl groups. The foregoing polysulfides may be represented by the following general formula:
wherein R and R1 may be the same or different hydrocarbon groups, and x and y may be integers from 0 to 8, and the sum of x and y is at least 1. A process for preparing such compounds is described in U.S. Pat. Nos. 2,719,125; 3,087,932; and 2,749,311. The reactions referenced and described in the foregoing documents may produce some amount of the by-product 2-hydrocarbyldithio-5-thiol-thiadiazole (not encompassed by the above-formula) as well as the 2,5-bis(hydrocarbyldithio)-thiadiazole compound both of which are DMTD compounds. The ratio of the two may be adjusted by varying the amounts of the reactants. However, a problem has been observed in that transmission fluids containing these DMTD compounds may prematurely darken, even though the wear characteristics may not yet have diminished in the transmission fluid. The typical user who notices the darkened fluid assumes that the transmission fluid is no longer functioning properly, and may prematurely replace the darkened transmission fluid.
An object of the present invention is to overcome this discoloration problem by including one or more thiadiazole compounds in a transmission fluid composition which will provide sufficient antiwear properties at extreme pressures while also having the advantage of color stability.